SLI = great improvements in HI RES GAMING only. And I'm talking 1900x1200+ resolutions only. Any less than that and, depending on the SLI strategy used by the driver, one card will sit there needlessly grabbing power and generating heat. So, the short answer is no
As you will be running a 750 chipset on your board, probably sticking with either a GTX 280 or a 9800GTX should be heaps for what you are doing. the money you save in not SLI'ing cards you can put towards one of those puppies. For reference, my 8800GTX more than handles every game available, including Crysis, which is playable at high res and medium to high quality levels. The 9800GTX is essentially on par in terms of performance with the 8800GTX, and the 280 is a fair bit ahead.
What power supply are you running? Brand and model? Thats more important than just the wattage, because any of those two cards will need the juice to work.
The reason I don't suggest the 4870 in this case is that you are running the 7xx-nVidia chipset. Its best practise in Performance PC design to never mix ATi and nVidia, as you can get driver conflicts, stability issues and performance issues. Any other chipset, and the ATI would be my recommendation.
